The invention relates to a method of spectral analysis, especially for a digital processing FM/CW type radio altimeter.
In these altimeters, a recurrent signal modulated in a linear ramp of frequency, known as a sawtooth frequency, is transmitted towards the ground. After reflection on the ground, the received signal is mixed with the transmitted signal. The frequency difference between the two signals, namely the beat frequency, is directly proportional to the propagation time (hence to the height) and to the slope of the frequency ramp.
Given the complexity of the phenomenon of reflection and the many back-scattered reflections that are picked up, it is actually a set of offset signals with different amplitudes and frequencies that is received. The received signal is therefore in fact a composite signal, and it is on the basis of the spectrum of the beat signal that the flying height can be determined.
The invention can be applied to the spectral analysis of the beat signal in a radio altimeter of this type, but nevertheless is not limited to this particular application.
This spectral analysis is presently achieved by digital means that give greater precision, improved discrimination of the echoes received, etc. To enable the processing of this signal, the beat signal is sampled in a known way.
A particular difficulty arises however in the case of low flying heights. Indeed, to keep the beat signal in a frequency band limited to a few hundreds of kilohertz and therefore avoid an excessively great dynamic range that would make it difficult to obtain a uniform analysis of the signal, the period of the sawteeth is subjected to a control loop related to the flying height. For a height varying from a few feet to 60,000 feet approximately, the period of the sawteeth varies from approximately 64 .mu.s to 100 ms, giving a dynamic range of about 10.sup.4, so as to keep either a constant beat frequency of 25 kHz or 100 kHz or a frequency contained within very narrow limits, for example between 60 kHz and 100 kHz. In the latter case, if the beat frequency tends to go out of this zone, it is brought back by the control loop.
For major heights, several hundreds of thousands of samples are available per sawtooth, owing to the fact that a high sampling frequency, generally equal to 256 kHz, is used. Several known methods of spectral analysis may be applied satisfactorily to obtain the desired result: these are the Fast Fourier Transform, the Fourier Transform of a self-correlation function or again other parametrical methods of the least error squares type using algorithms such as Burg's algorithm applied directly to the samples or Levinson's algorithm applied to self-correlation coefficients.
By contrast, for the small heights, the number of samples becomes very small: at most, for a 64 .mu.s sawtooth and a sampling frequency of 256 kHz, it is about 16 samples in theory. In practice, it is even smaller, in the range of 10 useful samples at most, and indeed several samples are lost owing to the delay given by the echo.
To cope with this difficulty, a different method of spectral analysis may be used for the small heights. This method is generally based on adaptive estimation methods such as the method of the gradient or the recursive least error squares method which assesses linear prediction coefficients that are re-estimated at each sample. This procedure however suffers from low reliability when the signal-to-noise ratio is small and does not accept breaks in phase between two successive sawteeth, namely between the last sample of one sawtooth and the first sample of the next sawtooth.
The French patent FR-A-2 650 413 describes a circuit implementing the various methods of spectral analysis mentioned here above with a system enabling the selection, at request, of a method of analysis such as the discrete Fourier transform, the gradient method or the Levinson method and the parameters for the implementation of the method selected.
This approach, while it enables the choice of a method more appropriate to spectral analysis in the case of small heights, does not however overcome the drawbacks inherent in this method, especially its unsatisfactory behavior in the presence of a poor signal-to-noise ratio.